For those that want to go a little more on the cheap, eVGA has a nice, stackable USB device that allows for multiple monitors. I blogged this a while ago: http://pitadeveloper.com/blog/post/Some-Things-Are-Worth-Paying-For.aspx

As far as I'm concerned, you can never be too rich, too thin, or have too much screen space. By "screen", I mean not just large monitors, but multiple large monitors. I've been evangelizing multiple monitors since the dark days of Windows Millennium Edition: Multiple Monitors and Productivity...